Nitrous oxide reductase (N2OR) is a homodimeric protein containing one dicopper site ("CUA") and one tetracopper site ("CuZ") per subunit. It is responsible for the catalytic two-electron reduction of N20 to N2 and is an important respiratory enzyme involved in ATP synthesis. The CuZ site of N2OR is proposed to be the catalytic site of the enzyme, yet little is known about the mechanism of reduction or oxidation states required for catalysis. The CuZ cluster contains a distorted tetrahedral ?4-sulfide bound to four copper atoms and is the only example of a N-sulfide tetranuclear copper cluster observed in nature. The goal of this research proposal is the synthetic design of novel model compounds, which mimic the structural arrangement, spectroscopic properties, and reactivity of the CuZ site in N2OR. Comparisons between the model compounds and the natural enzyme will be made to better understand certain properties, such as the unusual spectroscopic properties, redox behavior and reactivity, associated with the enzyme. The generation of mixed valence synthetic models that resemble the different forms of N2OR will be examined as well. [unreadable] [unreadable]